The present invention relates to a bundle and/or aggregate of short carbon fibers (hereinafter referred to as short carbon fiber bundling mass), a fiber-reinforced thermoplastic resin composition obtained by mixing the short carbon fiber bundling mass with a thermoplastic resin to be able to serve as a metal substitute under high temperatures, and a process for producing the short carbon fiber bundling mass.
Recently, fiber-reinforced plastic compositions prepared by mixing or dispersing carbon fibers in various kinds of matrix are attracting attention as an industrially important material because of their excellent mechanical properties such as high strength, high rigidity, low specific gravity and high wear resistance. A resin highly improved in heat resistance, so-called super-engineering plastics is developed and it is expected that a new type of carbon fiber-reinforced resin composed of such new resin and carbon fiber is used in a new and wide scope field as a metal substitute under high temperatures.
Generally, in the preparation of fiber-reinforced resin materials by mixing or dispersing carbon fibers in various kinds of resin, when short carbon fibers are mixed in the form as they are in the resin, there arise various problems in the step of mixing or dispersion in the matrix resin. For instance, there are formed the pills of short carbon fibers in the course of their supply by a feeder, and such pills tend to cause blocking in the feed step or prevent the fibers from being uniformly dispersed in the matrix. For facilitating handling of short carbon fibers to enhance workability in the dispersion step, there are used the fiber bundles or aggregates (fiber bundling masses) formed by bunching a plurality of short carbon fibers with a sizing agent.
However, when the conventional short carbon fiber bundling masses are mixed with a resin, and then heated and molded to form a fiber-reinforced plastic molding material, gases may be generated, thereby impairing the working environment or voids may be formed in the molded product. Particularly in the case of using the super-engineering plastics having high heat resistance, since its molding temperature is high because of its high heat resistance, generation of gases is encouraged and a great many voids are formed in the molded product.
In the circumstances, there has been strong request in the art for the short carbon fiber bundling mass which, blended with a resin, enables to produce the molded products which are composed of the short carbon fiber bundling mass and the resin and are free of voids, and which have little risk of generating gases in the molding operation.
As a result of the present inventors' strenuous studies for solving the above problems, it has been found that based on the assumption that generation of gases during molding operation under heating is principally due to thermal decomposition of a sizing agent, by carbonizing a bundle of short carbon fibers with the sizing agent in an inert atmosphere at a temperature of 400.degree. to 1,500.degree. C., the thus-obtained short carbon fiber bundling mass comprises not less than 90 wt % of short carbon fibers and the balance substantially consisting of carbonized materials derived from a resin as the sizing agent, wherein a weight loss of the short carbon fiber bundling mass in an inert atmosphere at a temperature of 400.degree. C. is not more than 0.5%, the short carbon fiber bundling mass is easily dispersed in a resin when mixed therewith while maintaining the form of the bundling mass, and there takes place substantially no generation of gases during the molding operation, thereby preventing formation of voids in the molded product. The present invention was achieved on the basis of this finding.